OBJECTIVE: (1) To develop hormones assay for measurement of testosterone and cortisol in male muriquis. (2) To determine if male muriquis exhibit seasonality in the their levels of testosterone and cortisol. (3) To examine the source of seasonality in male muriquis . RESULTS Seasonal changes do occur in both testosterone and cortisol levels in wild male muriquis but these changes are not prior to the onset of sexual behaviors as is seen in several other primate species. Fecal cortisol and testosterone concentrations were measured in six male muriqui monkeys living in multi-male-female societies without observable competition for females. These males were living totally undisturbed at the Esta[unreadable] o Biologica de Caratina in Minas Gerais, Brazil in a group of more than 60 muriquis. All male muriquis showed seasonal steroid changes with mean testosterone levels increasing an average of 2.4 + 0.98 days following the first observed copulation of the 1996 breeding season while cortisol levels increased an average of 12.2 + 3.77 days following the testosterone increase. Testosterone levels decreased before the end of the copulation season whereas cortisol levels remained elevated until the end of the copulation season. In the following year, the copulation season did not end with many of the muriquis continuing to copulate throughout the usual noncopulatory season. Under these conditions, testosterone levels declined as in the year before but then rose again within a month and remained elevated. Cortisol levels remained elevated throughout the second year. Comparisons of male steroid profiles during the same months over consecutive years implicate both ecological and social factors in regulating male muriqui reproductive seasonality. Visual inspection and hormonal patterns do not indicate a fattening response in male muriquis as is seen in the squirrel monkey or the rhesus monkey. FUTURE DIRECTIONS We plan to examine both male and female muriqui reproductive seasonality simultaneously to better understand the social and environmental factors influencing hormone release. KEY WORDS seasonal reproduction, fecal testosterone and cortisol, wild muriqui monkeys, New World primate